Schlieren imaging is well known. Schlieren imaging is a technique for visualizing wavefront distortions or “phase objects” in transparent media. Schlieren imaging can be used to characterize such physical parameters as temperature, density, and pressure. As those skilled in the art will appreciate, such parameters are important in aerodynamics, fluid dynamics, and various other branches of engineering. Schlieren imaging facilitates the formation of an image using light rays that pass by a sharp cutoff filter. The filter is arranged so that the ray intensity has a steep derivative along the edge of the filter. This arrangement allows small deviations in the path of the light rays to produce a large change in the intensity of an image with respect to the intensity of the image caused by undeviated light rays.
Classical Schlieren systems use light collimated by optics such as mirrors or lenses. The use of such optics substantially limits the area under test to approximately the size of the optics. An important improvement with respect to the original Schlieren concept is the complementary-grid focusing Schlieren system. The complementary-grid focusing Schlieren system uses the same edge filtering. However, instead of using collimated light, the complementary-grid focusing Schlieren system images a background grid pattern onto a complementary opaque cutoff filter such that the cutoff filter edges lie along the edges of the background grid pattern. The target object is imaged in a different plane with respect to the cutoff filter, as facilitated by the camera objective. The focusing approach generally allows for obtaining Schlieren imaging through much larger areas without using excessively large optics. Such techniques thus generally require less expensive equipment as compared to classical collimated-light Schlieren systems and can more easily be scaled up to cover large areas. However, such techniques tend to be more difficult to set up and align as compared to classical collimated-light Schlieren systems.
Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.